This invention relates to electric power transmission and distribution equipment, and more particularly to devices for controlling the electric field in apparatus for terminating or joining certain types of electric power transmission cables.
Electric power is often transmitted at voltages exceeding 50 kV in order to reduce power losses caused by the resistance of the conductors. Traditionally, such high-voltage conductors have been suspended high above the ground from towers or other suitable supports in order to isolate them from the ground and from other objects where a high difference of potential would exist between the conductors and the objects. In such applications, the conductors are electrically insulated from the supports by suitable insulator apparatus, and from everything else by the air present in the region around the conductor. As is well known, an electric field surrounds the conductor. Because air has a relatively low dielectric strength, the conductors must be separated from other objects by a relatively large distance to prevent the electric field gradient in the region between the conductor and the object from exceeding dielectric strength of the insulating air.
The above-ground transmission of electric power via suspended conductors may be inappropriate for certain applications. In some cases, the requirement that the conductors be spaced far from other objects is inconsistent with existing or planned land use patterns. In other cases, aesthetic considerations preclude the use of the large towers or other supports required. One possible solution to this problem would be to locate air-insulated conductors underground in suitable vaults, but the need to maintain adequate physical separation between each conductor and other conductors and surrounding objects would require huge vaults and renders this solution economically infeasible.
For these and other reasons, systems have been designed to permit electricity to be transmitted at high voltages through suitable cables having configurations which do not require large physical spacing between the conductor and other objects. In one such cable configuration, a center conductor is surrounded by a layer of an appropriate solid dielectric material, such as polyethylene. The solid dielectric layer is, in turn, surrounded by a conductive shield. The center conductor, the solid dielectric, and the conductive shield are concentrically disposed. The center conductor has a substantially circular cross section. In order to avoid skin effect, the center conductor may comprise several groups of smaller conductor strands. Such groups are arranged as sectors of the circular center conductor cross section. The conductive shield may be formed as a tubular layer of partially conductive material having one or more drain conductors running along the outside surface of the layer. A corrugated metal tube or other suitable armor may be provided around the conductive shield to provide physical protection against damage to the cable.
The solid dielectric layer is formed from a suitable material having a high dielectric strength to minimize the distance required between the center conductor and the shield for a given operating voltage. This reduces the amount of material required to construct the dielectric layer and all other layers disposed radially outward from the dielectric layer. Accordingly, the weight, cost, and overall diameter of the cable is minimized.
Although the cable configuration described above provides a variety of advantages, special care must be taken when the cable is joined or terminated, because the high dielectric strength of the solid dielectric material permits a conductor-to-shield separation in the cable which is typically a small fraction of the separation required to prevent breakdown in air. When a cable is terminated (e.g. when it is desired to use the cable to feed an overhead line, or vice versa), the shield conductor must end. However, because the conductor-to-shield separation is small, an abrupt end to the shield causes large electrical stresses which would cause breakdown if not suitably controlled. A similar situation exists when it is necessary to join two cables together, because the shield conductor cannot conveniently be made continuous across the joint.
Accordingly, joints or terminations applied to solid-dielectric cables are typically immersed in a suitable container of insulating fluid (e.g. oil), having a high dielectric strength in order to reduce the separation required to avoid breakdown. In addition, conductor arrangements are chosen carefully to avoid sharp edges and other configurations which produce concentrations of electrical stress and thereby promote breakdown.
In the past, stress control cones have been constructed from conductive materials to help alleviate these problems. In one known design, a funnel-shaped metal casting is attached at its wider end to a larger-diameter epoxy cylinder. A partially conductive material is applied to at least a part of the inner wall of the epoxy cylinder and extends to contact the metal casting. An end of the cable to be protected is prepared by removing a length of the conductive shield to reveal the solid-dielectric insulation. The cable is inserted through the stress cone such that the shield ends within the cone, and the metal casting is electrically connected to the shield conductor. The partially conductive material forms an extension of the funnel-shaped metal casting, so that in effect, the diameter of the extended shield conductor is gradually increased through the metal casting and the partially conductive material. By gradually increasing the distance between the center conductor and the shield at the region where the shield terminates, the electric field gradient in that region, and the resulting electrical stress, are reduced.
However, stress cones constructed according to the prior art suffer from a variety of problems. It is difficult and expensive to manufacture the cast epoxy cylinders. Because air has a low dielectric constant, air-filled voids in the epoxy will permit breakdown. Certain contaminants may also affect the dielectric strength of the epoxy. Because it is difficult to prevent such voids and contamination from occurring, many castings must be rejected.
It is also difficult to securely join the epoxy cylinders to the metal castings. In joint applications, in which the longitudinal axis of the stress cone is typically disposed parallel to the ground, and in which the stress cones are subject to relatively large temperature variations, the castings may separate from one another, resulting in possible failure of the joint.
Another problem in manufacturing the prior art stress cones arises from the requirement that the conductive surface on the inside of the cone be extremely smooth. Because the seam which exists between the metal casting and the partially conductive material applied to the epoxy cylinder creates an inherent discontinuity in that surface, breakdown tends to occur at the seam. In addition, once the partially conductive material has been applied, a secondary machining operation is required to achieve the desired interior surface finish. The machining can cause the conductive material to chip or fracture, and therefore, still more assembled stress cones must be rejected.
Still another problem with the prior art stress cones is that the conductive metal casting and the partially conductive material are directly exposed to the insulating fluid. Contaminants in the fluid, and in particular metal particles suspended therein, are attracted to regions of high electrical stress and the adjacent conductors. When such a particle comes in contact with a conductor, it forms a sharp protrusion into the fluid. Such sharp protrusions cause concentrations of electrical stress which may exceed the dielectric strength of the fluid. In addition, such concentrations tend to attract other particles, resulting in progressively longer breakdown-promoting conductive chains.
Similar problems attend corona shields which are typically applied to the current carrying conductor to control the stress in a region near a part of the cable or an accessory thereto, which, due to its geometry, would tend to promote electrical breakdown if not so protected.
Other prior art stress control devices are known which comprise a conical metal conductor which is applied around a cable. The stress control device is then secured to the cable by applying a moldable potting compound in the interior of the conical conductor, producing a tight "interference" fit between the conical conductor and the shield or solid dielectric. A problem with these devices is that at elevated temperatures, they apply pressure to the cable which causes extrusion of the solid dielectric material, thereby undesirably reducing the distance between the center current-carrying conductor and the shield conductor, possibly causing a fault. In addition, once the solid dielectric material has been extruded from the interior of the cable, the interference fit between the stress cone and the cable is defeated.